Transmission with selector valves



May 20, 1969 K. F. GOLAN ET AL 3,444,762

TRANSMISSION WITH SELECTOR VALVES 7 Original Filed Feb. 1, 1966 Sheet of5 1 3:2; FOR J? 3 iii; (2) RE\/ 4TH G59 E1 E- JE KENNETH QQK ES DAVID S.OHAVER SHAIRYL I. PEARCE GERALD D. ROHWEDER May 20, 1969 GOLAN ET AL3,444,762

TRANSMISSION W[TH SELECTOR VALVES Original Filed Feb. 1, 1966 Sheet 3 of5 4O TORQUE HEAT CONVERTER EXCHANGER 32 35 27 I38 14 INVENTORS H KENNETHF. GOLAN DAVlD S. OHAVER SHAIRYL I. PEARCE GERALD D. ROHWEDER May 20,1969 GOLAN ET AL TRANSMISSION WITH SELECTOR VALVES Sheet 3 of5 OriginalFiled Feb. 1, 1966 7 h I5 I:

moT U U om @E Q 5 g g XNQ \N M v a. @Q 3 m2 3 m Q: L

INVENTORS KENNETH F. GOLAN DAVID S. OHAVER SHAIRYL I. PEARCE GERALD D.ROHWEDER K F. GOLAN ET AL May 20, 1969 TRANSMISSION WITH SELECTOR VALVESSheet Original Filed Feb. 1, 1966 F5035 mvzdm ZOEQEWZd NE.

NON mom @QN 09 m INVENTORS KENNETH F. GOLAN DAVID s. OHAVER SHAIRYL. I.PEARCE GERALD D. ROHWEUER May 20, 1969 K. F. GOLAN ET AL 3,444,762

TRANSMISSION WITH SELECTOR VALVES Original Filed Feb. 1, 1966 Sheet 5 of5 TOTAL :55a

TIME 40o RATE RATE 1.? A I IL E g 300 p I SPEED CLUTCHES LU A (4,.SAND6)5 200 D m Lf) I LU B/ D|REET1DNAE CLUTCHES d 100 (r, 2 AND 3) CL I OL-J- I 4 TiME (SEQ) E .T. 5 E:

MODULATION 40o PEmoD l PRESSURE (D. s. s.)

INVENTORS KENNETH F. GOLAN DAVlD S. OHAVER SHAlRYL I. PEARCE GERALD D.ROHWEDER United States Patent US. Cl. 74-754 3 Claims ABSTRACT OF THEDISCLOSURE Friction clutches and brakes for changing the ratios of aplanetary transmission including first and second selector valveswherein the first valve when in a certain position directs fluid to ablocker means which prevents the second valve from moving to one of itspositions.

This is a divisional application of copending US. application Ser. No.523,980 of Kenneth F. Golan, et al., filed Feb. 1, 1966 for TransmissionHydraulic Control, and now Patent No. 3,389,770.

The present invention relates to a hydraulic control system for atransmission and more particularly to a control system for atransmission specifically designad to meet the requirements of wheelloaders and vehicles having similar operating characteristics.

The present invention is described herein with reference to atransmission which has been specifically designed to meet the needs of awheel loader but is not necessarily limited thereto. As will becomeclear from the following description, there are certain operatingcharacteristics associated with a wheel loader which make the presentinvention extremely advantageous for use therewith. It will beunderstood by those in the art however that the present invention isalso advantageously employed with vehicles other than wheel loaders.

Smooth forward to reverse shifts are a necessity on a wheel loader whichcontinually cycles in a loaded and unloaded condition. Not only isoperator and machine fatigue involved, but an abrupt transition fromforward to reverse can cause dirt in a heavily loaded bucket to spillonto the operator or the ground. In the latter case, the spilled dirtcauses the ground on which the loader operates to become uneven whichfurther disrupts operation, and in the end requires the production cycleto be interrupted or delayed until proper travel surfaces can berestored.

Due to the nature of the work performed by a wheel loader a broadspectrum of ranges is encountered relative to vehicle load and withvarious engine speeds, and it is necessary to specifically tailor thetransmission control to this range. With transmission clutches that arekept to a minimum size for purposes of economy, it has proven to beadvantageous to sequence engagement of the clutches by: (l) reducing theenergy absorbed in the clutches during full load shift by increasing theinitial rate of clutch pressure engagement relatively rapidly; and (2)controlling the final portion of clutch pressure engagement at a lowerrate. Alternately, a transmission which has a relatively large torquecapacity compared with the possible vehicle loads should have a controlwhich engages early in pressure rise or modulation portion of the cycleand, therefore a slow rate of pressure rise is initially desirable forsmooth clutch engagement since energy absorption is no problem. Afterthe relatively low rate of engagement of a large capacity transmission arelatively rapid rate is desired in order to reduce the modulationperiod, and hence the duration of interrupted power to the vehiclewheels.

In hydraulic control systems for transmissions presently known in theart it is not uncommon to provide a fixed pressure differential betweentwo clutches which must be engaged in order to condition thetransmission for driving the vehicle wheels. When a particulartransmission has design parameters which require a larger-than-normaldifferential valve setting a problem is created in that the initialdifferential pressure causes extremely rough shifts which, as mentionedabove, are to be avoided if at all possible. Since a transmissiondesigned to meet the needs of a Wheel loader includes parameters whichrequire a larger-than-normal differential valve setting, it is necessaryto provide a control system for the transmission which will eliminatethe rough shifts which such a largerthan-normal differential valvesetting produces.

Transmissions for which the present invention are specifically providedrequire means making it unnecessary for the operator of the vehicle toaccurately feel or maneuver the transmission control lever under therelatively rapid cyclic conditions of such a vehicle. Thus, the presentinvention provides an over-shift device to allow rapid first to secondshifting while decreasing the possibility of an over-shift to third.Further, while a fourth forward speed is available the configuration oftransmission does not provide a fourth reverse speed, and thus the rapidforward-reverse lever operates to automatically downshift the speedselector lever from fourth to third position when the transmission isshifted from forward to reverse while in fourth gear. It also preventsplacement of the speed selector lever in fourth position while inreverse.

Accordingly, it is an object of the present invention to provide ahydraulic control system for a transmission which is designed foroperation with a vehicle such as a wheel loader wherein the controlsystem produces smooth operation and uncomplicated gear shifting whichenables the vehicle operator to direct his attention to the loading andunloading of the vehicle rather than the shifting of the vehicle.

A specific object of the present invention is to provide a hydrauliccontrol system for a transmission wherein the control system includes amodulation valve for varying the pressure directed to a pair of selectedclutches, plus a differential valve for maintaining a pressuredifferential between the two clutches selected wherein the pressuredifferential valve produces a variable pressure differential over aportion of the modulating cycle.

Further objects of the present invention include providing a novel dualrate modulation valve means including a piston operated relief valve, anovel differential valve means, and a dual purpose safety valveadvantageously used in conjunction with a transmission neutralizingsystem.

Further and more specific objects and advantages of the presentinvention are made apparent in the following specifications when takentogether with the drawings.

In the drawings:

FIG. 1 is a schematic illustration of a transmission of the general typefor which the present invention is designed;

FIG. 2 is a diagrammatic illustration of the shift pattern forconditioning the transmission of FIG. 1 to one of the four forwardspeeds or one of the three reverse speeds or neutral wherein the numberswithin the circles designating shift positions refer to the transmissionclutches which are engaged to produce the desired gear ratios;

FIG. 3 is a diagram showing how FIGS. 4, 5 and 6 are combined to form asingle semi-schematic illustration of the control system of the presentinvention;

FIGS. 4, 5 and 6 are portions of a composite drawing showing insemi-schematic form the control system of the present invention;

FIG. 7 is a pressure versus time graph illustrating the pressure risesproduced in selected clutches by the control system of the presentinvention;

FIG. 8 is a pressure versus time graph illustrating alternate pressurerises which are achieved with a modified form of the present invention;and

FIG. 9 illustrates the modification of the control system of FIGS. 4, 5and 6 which is necessary in order to produce the pressure rises of FIG.8.

Referring now to FIG. 1, a transmission comprising several epicyclicgear sets is conditioned to transmit torque at various gear ratiosdepending on which of the several friction engaging means (clutches andbrakes) are engaged. The numerals 1-6 in the drawing refer to thefriction engaging devices appearing directly therebelow whereby it ispossible by referring to FIG. 2 to determine which of the frictionengaging devices are engaged to produce the several forward and reversegear ratios. Friction engaging devices 1, 2 and 3 are referred to asdirectional (or load) clutches, while the friction engaging devices 4, 5and 6 are referred to as speed clutches. Each gear ratio is formed byone of the load clutches and one of the speed clutches being engaged;unless one clutch from each set is engaged power will not be transmittedthrough the gear train. The directional clutches are often referred toas the load clutches as the selected one of the load clutches isconditioned to be engaged after the selected speed clutch and thusoperates to pick up the load. The manner in which the control systemoperates to produce this result will be fully explained below.

Referring now to FIGS. 4, 5 and 6 the main compo nents of the controlsystem of the present invention include valves 11 and 12, thespools ofwhich are linked together (by linkage not shown) for selecting one ofthe speed clutches and one of the load clutches through movement of asingle speed control lever (not shown), a directional selector valve 13for selecting a load clutch through operation of a separate directionalcontrol lever (not shown), a modulating relief valve 14, a pressuredifferential valve 16, a safety valve 17 and a flow limiting valve 18. Apump 21 draws fluid from a reservoir 22 and delivers it through a filter23 to valve 18 from which a portion is directed to the modulating reliefvalve 14. The fluid controlled by modulating relief valve 14 is directedto the selected speed clutch for engagement thereof, and also to thepressure differential valve 16 via safety valve 17. The pressuredifferential valve 16 reduces the pressure of the fluid controlled bythe modulating relief valve 14 and directs it to the selected loadclutch. The speed clutches are on the upstream side of the differentialvalve 16 and the load clutches are on the downstream side, thusassurring a pressure differential between the engaging forces acting onthe selected clutches.

One of the outstanding features of the present invention resides in thepressure differential valve 16 in that it does more than simply producea fixed pressure differential or a proportional pressure differential asis already known in the art. As will be more fully described below, thevalve 16 operates to produce a relatively small pressure differentialduring the beginning of a clutch engagement cycle, then graduallyincreases the differential pressure as the modulation period continues(valve 14 increases its regulated pressure), and upon reaching apredetermined pressure maintains a fixed differential (which isconsiderably larger than the initial pressure differential) over theremainder of the modulation cycle.

The graph of FIG. 7 clearly illustrates the relation ship between thespeed clutch fluid pressure (shown as a solid line) and the load clutchfluid pressure (shown as a dashed line). The details of this graph willbe more fully described following a detailed description of thecomponents which comprise the control system of the present invention.

Flow limiting valve 18 comprises a piston 26 slidably disposed in a bore27 and urged leftwardly by a spring 28. Fluid from filter 23 is directedthrough conduit 31 to a recess 32 surrounding bore 27 at the leftwardlymost end thereof. Fluid flows out of recess 32 through a restrictiveorifice 33 which communicates with a passage 34. This fluid is alsocommunicated with bore 27 and the chamber of spring 28 through suitableorifices 29 in piston 26. When the fluid flow through orifice 33 exceedsa predetermined rate, the pressure differential created by orifice 33operates to compress the spring 28 and urge the piston 26 rightwardlyallowing the excess fluid to flow to a passage 35 which leads to thetransmission torque converter 40.

Flow limiting valve 18 is vitally important in systems requiring highpump flow since the modulation rates necessary for proper operation ofthe transmission depend on a source of fluid which is not excessive.

The output from orifice 33 is directed to the modulating relief valve 14through the passage 34 which is in communication with a recess 36 whichsurrounds the bore 37 of the modulating relief valve. Disposed withinthe bore 37 is a valve spool 38 which is urged in a leftward directionby a spring 39 one end of which is connected to the valve spool 38, andthe other end of which is connected to a piston 41 which is alsoslidably disposed within a bore 40 which is slightly larger than bore37. A land 42 on spool 38 cooperates with a recess 43 to direct fluidfrom recess 36 to a passage 44 which leads through valve bore 27 tolow-pressure passage 35. Thus, when the pressure of the fluid in recess36 is suflicient to urge the spool 38 rightwardly against the spring 39a distance suflicient to establish communication between recess 36 andrecess 43, further pressure buildup will only be possible by increasingthe force with which spring 39 opposes the rightward movement of spool38. Since piston 41 is slidably disposed within the bore 37, thepressure in recess 36 is made variable by controlling the position ofthe piston 41 and thus the force with which spring 39 urges spool 38leftwardly.

Recess 36 communicates with a clutch supply passage 46 via a passage 47.Passage 46 directs the fluid therein to the bore 48 of selector valve 13at two separate longitudinal positions via branch passages 49 and 51.Depending on the positions of the selector valve spools 52, 53 and 54 ofselector valves 11, 12 and 13 respectively, one of clutches 4, 5 and 6will receive fluid from passage 46 while the other two clutches will becommunicated with a drain and thus disengaged. The selector valve spoolsare illustrated as positioned to establish a third gear forwardcondition which is established by the engagement of clutches 3 and 4.Fluid is directed to clutch 4 via passage 51, selector valve bore 48,passage 56, selector valve bore 57, passage 58 and valve bore 48.

At the same time that the fluid controlled by modulating relief valve 14is delivered through passage 46 to one of the speed clutches, it alsodelivers fluid to the pressure differential valve 16 via passage 47,safety valve bore 61, and passage 62. Fluid in passage 62 is directed toone end of a valve bore 63 in differential valve 16 through a recess 64which surrounds the bore 63. Disposed within valve bore 63 is a valvespool 66 which is urged leftwardly in the bore by a spring 67 one end ofwhich is seated in the spool 66, and the other end of which is seated ina piston 68 which is slidably disposed within the bore 63. The end ofvalve spool 66 adjacent to recess 64 has two sets of ports 71 and 72formed therein which lead to a hollow interior. Fluid entering valve 16from passage 62 enters the interior of spool 66 through ports 71 andoperates to urge the spool rightwardly against the spring 67. When thepressure of the fluid in passage 62 is sufficient to counteract thespring 67 and move the spool rightwardly, ports 72 register with arecess 73 which surrounds the bore 63 and which communicates with apassage 74 via a branch passage 76. Passage 74 communicates with thebore 48 of selector valve 13, and depending upon the positions ofselector valve spools 52, 53 and 54, one of the load clutches receivesthe fluid in passage 74. As previously mentioned, the valve spools areshown positioned to effect a third gear forward condition which callsfor load clutch 3 to receive fluid. The communication between clutch 3and passage 74 is effected through bore 48, connecting passage 77,recess 78 and valve bore 79 and selector Valve 12.

At the same time that fluid is being provided to one of the loadclutches via passage 74, fluid is also being directed to a check valve81 through a passage 82 one end of which communicates with passage 74and the other end of which communicates with a valve check bore 83through a recess 84. When fluid enters bore 83 it causes a floatingcheck valve piston 86 slidably disposed therein to move rightwardly andcontact a stop 87. In this rightward position the piston 86 blockscommunication between bore 83 and a drain 88 thus making it possible forfluid pressure to build up in bore 83. When the drain 88 is effectivelydisconnected from the bore 83 fluid entering the recess 84 from passage82 is directed to a passage 91 which communicates with a passage 92. Oneend of passage 92 leads through an orifice 93 to the bore 37 ofmodulating relief valve 14. The other end of passage 92 leads through anorifice 94 to an annular chamber 96 which communicates with a passage 97that leads to a chamber 98 behind the modulating piston 41.

The modulating piston 41 includes an annular recess 99 which registerswith the orifice 93 when the piston 41 contacts an end stop 101. Whenrecess 99 and orifice 93 register fluid is able to flow into chamber 98from both passage 92 and passage 97 whereby the pressure buildup inchamber 98 is of a particular rate giving rise to a particular rate ofpressure increase in passage 47. As the piston 41 moves leftwardly therecess 99 moves out of register with orifice 93 whereby the only sourceof fluid in chamber 98 is through orifice 94. In this way two differentrates of pressure increase in passage 47 are effected.

Each time that a gear shift is effected, the pressure in passage 74 willdrop during the period required to fill the newly selected clutch, andthus the pressure in bore 83 will also drop allowing the piston 86 tomove leftwardly and uncover the drain 88. With the drain 88 uncoveredthe fluid in chamber 98 behind piston 41 drains quickly whereby thespring 39 urges the piston 41 against the stop 101, which is thestarting point for each new modulation cycle.

From the foregoing description it is seen that as the pressure inpassage 47 increases after the filling of a selected speed clutch, thereis an accompanying increase in the pressure at the output ofdifferential valve 16 which results in the piston 41 moving leftwardlymaking it possible for even greater pressure increases to occur inpassage 47 from modulating relief valve 14. If the differential valve 16were designed to have a fixed pressure differential between its inputand output, the difference between the fluid pressure at the speedclutch and the fluid pressure at the load clutch would be fixedthroughout the entire modulation cycle. The pressure differential acrossthe valve 16 is not fixed, however, but varies depending upon thelongitudinal position of a modulating piston 106 slidably disposedwithin a bore 105 which is slightly larger than the bore 63 of valve 16.A chamber 107 defined in part by an end stop 108 receives pressure fluidfrom a passage 109 which is connected to passages 111 and 112, one ofwhich carries fluid corresponding in pressure to that regulated bymodulating relief valve 14. Thus, the pressure differential which thevalve 16 establishes between the selected load clutch and the selectedspeed clutch is a function of the regulated pressure of modulatingrelief valve 14 and thus the pressure in the selected speed clutch.

The speed clutch pressure which is communicated to the left end of valvespool 66 urges the spool righwardly and is opposed by spring 67 andfluid at load clutch pressure which enters the spring chamber 60 viarecess 73, bore 116 and port 117. Spool 66 is biased rightwardly byforce acting on the valve due to pressure in chamber 64, and leftwardlyby forces provided by spring 67 and pressure in chamber 60. Theseopposing forces are substantially equal during regulation and aretransmitted to piston 68. Therefore, the pressure in chamber 107, whichis equal to the pressufe in chamber 64, would not initially move piston106 leftwardly unless its area would exceed that of piston 68. As thepiston 106 moves leftwardly it causes spring 67 to compress and thusincrease the force urging the spool 66 leftwardly thereby increasing thepressure difference between passage 76 and chamber 64. While the spring67 is selected to transmit high forces and determines what pressuredifference is obtained, a spring 121 surrounding the end of piston 106operates to transmit relatively light compensating loads and is selectedto be of a size which provides that the desired maximum differential isobtained when the piston 68 abuts a stop 122 with speed clutch pressureat a predetermined value. Thus, the differential pressure may vary, forexample, between 20 p.s.i. when the piston 106 abuts the stop 108, and75 p.s.i. when the piston 68 contacts stop 122. By proper selection ofthe spring 121, the piston 68 contacts the stop 122 and thus the maximumpressure differential is reached at approximately the same time that thepiston 41 blocks the orifice 93 and the rate of modulation produced byvalve 14 changes. During the remainder of the modulation cycle, valve 14increases its regulated pressure at a rate determined by the flow offluid through passage 97. Thereafter the differential pressure betweenthe fluid directed to the speed clutch and the fluid directed to theload clutch is maintained constant.

FIG. 7 graphically illustrates the operation described above whichproduces the operating characteristics which were initially set out asthose most desirable for operation in conjunction with vehicles such aswheel loaders. Lines A and A represent steady state operating pressuresof the speed clutch and directional clutch respectively. When a shiftoccurs the clutches are quickly drained and the newly selected clutchesbegin to fill at the pressures B and B. Once the clutches are filled themodulation cycle begins and the pressure to the speed clutch anddirectional clutch increases at the rate indicated by lines C and C. Atthe points D and D the maximum differential is achieved (it is to benoted that the maximum differential represented is aproximately 75p.s.i. while the initial differential is aproximately 20 p.s.i.) and theremainder of the modulation cycle is at a different rate of pressureincrease (as indicated by lines E and E) with the differential pressureremaining substantially constant. While the particular modulation ratesillustrated in FIG. 7 have been found advantageous for actual operationwith a transmission for a wheel loader, the invention is not limited tothis single configuration but can be easily altered to suit the needs ofa given vehicle.

FIG. 8 illustrates in graphic form a modified modulation configurationwhich includes a first rate of modulation with a variable differentialpressure followed by a faster rate of modulation at a fixed differentialpressure. FIG. 9 illustrates a portion of a modified modulating reducingvalve 14' which produces the pressure trace of FIG. 8. A recess 99 inthe piston 41' is located such that an orifice 93 is blocked from thecompartment 98' behind piston 41' for all but the last portion of themodulation cycle, and thus only contributes to the fluid flow providedthrough orifice 94 during the latter mentioned portion of the modulationcycle.

A pressure ratio valve 131 operates as the torque convertor inlet reliefvalve to aid in producing smooth shifts.

In addition, speed clutch pressure is routed to a slug cavity 132 in thevalve 131 via passage 133, annulus 134 and a port 136 in the valve spool137. Torque convertor inlet pressure is routed to the poppet or left endof the spool 137 by means of an intercommunicating passage 138, annulus139, port 141, opening 142, and a poppet 143. In the particular exampleillustrated the cross-sectional area relationship between a slug 144associated with cavity 132 and spool 137 is approximately .39 to 1. Thismeans that for every ten p.s.i. of speed clutch pressure there is only3.9 p.s.i. torque converter inlet pressure. Any pressure to the torqueconvertor in excess of this ratio will move valve spool 137 rightwardlyand fluid will be bypassed to the transmission lubrication system viapassage 146. When a shift is made the speed clutch fluid drops to fillpressure and ratio valve spool 137 moves to the right allowing torqueconvertor inlet fluid to be dumped to the lube circuit thus lowering theconvertor inlet pressure. This reduced back pressure insures the startof modulation as established by the modulating relief valve initialsetting. Since the torque convertor does not transmit much torque withthe lower pressure the shift is smoother.

The safety valve 17 is provided to prevent the vehicle from beingstarted while the transmission is in any gear other than neutral. Whenthe vehicle is shut off (no pump pressure) a spring 147 forces a valvespool 148 rightwardly in a bore 149 until it contacts a stop 151. Whensafety valve 17 is in this position, communication is prevented betweenconduit 47 and differential valve 16 thus preventing the load clutchesfrom being engaged. In order to move spool 148 leftwardly in order toestablish the necessary communication for energizing a load clutch, theselector spools must be positioned to neutral. When in neutral fluid isrouted from selector valve 13 through line 152 to the safety valve 17,and more particularly to the right end thereof. Only when neutralposition is chosen does fluid flow in line 152 since the line iscommunicated to a drain whenever forward or reverse is selected. Thefluid in line 152 is communicated to a cavity 153 via an annulus 154, aport 156 within safety valve spool 148, and a poppet 157. The poppetenables fluid to freely enter the cavity 153 from line 152 but operatesto restrict the flow of fluid out of the cavity thus preventing thesafety valve from resetting during a normal shift.

As previously mentioned, one of the objects of the present invention isto provide a means for preventing the transmission from being placed ina fourth speed reverse. In order to accomplish this result a shiftblocker 178 is provided. Any time that the spool 54 is positioned toselect reverse, fluid is routed from bore 48 by means of a passage 180to a cavity 182 at the left end of blocker 178. With such pressurecommunicated to cavity 182, the blocker moves towards speed selectorspool 53 and prevents the spool from being moved leftwardly into thefourth speed position. When spool 54 is positioned for forward drive,however, fluid behind the blocker 178 in cavity 182 is communicated to adrain allowing the fourth speed position to be selected if desired. If aforward to reverse shift is made While in fourth gear, the controls willautomatically change the position of selector 53 to that correspondingto third gear. To prevent the inertia of the speed selector linkage andspools from downshifting past third, a damping orifice 184 is providedbetween the spool 52 and the drain 185. Damping is obtained by forcingfluid out of the end of bore 57 as the spool 52 moves to the right fromthird to second gear. This arrangement allows a normal undamped shiftfrom fourth to third, and a damped shift out of third into second.Because of an enlargement of bore 57 and its relationship to a spoolland 186, orifice 184 is bypassed during shifts from second to firstgear and no damping is provided.

In addition to the blocker 178 there is associated with spool 53 aninhibitor piston 188 between shift blocker 178 and spool 53 to preventovershifting into third when shifting from first to second The piston188 is biased leftwardly by a spring 190 seated at one end in the piston188 and the other end to a step 192 in the valve bore. When the selectorvalves are positioned for first speed, fluid is routed behind the piston188 causing it to move towards the spool 53. Fluid is presented behindthe piston 188 as a result of the fact that clutch 6 is pressurized inboth first speed forward and first speed reverse and an interconnectingpassage 194 communicates clutch 6 with a cavity 196 between blocker 178and piston 188. With piston 188 against stop 192, spool 53 may beshifted leftwardly into second gear without contacting piston 188. Withany shift to second gear fluid behind the piston is connected to drainthrough a pair of orifices 197 and 198 such that piston 188 isrestricted to slow leftward movement under the influence of spring 190.Should the operator attempt a rapid first to third shift additionaleffort will be required between second and third position of spool 53because of the restriction of orifices 197 and 198. This serves tominimize operator attention for shifts between first and second byreducing the possibility of an inadvertent shift beyond second gear.

A transmission neutralizer valve 201 includes a valve spool 202 slidablydisposed in a bore 203 and urged righwardly in the bore by a spring 204.The rightward end of the bore 203 communicates with a brake-actuatedpneumatic system (not shown) through a conduit 206 whereby the spool 202is urged leftwardly against the spring 204 by air pressure when aparticular brake pedal of the vehicle is operated. When this occurs, thecavity 153 behind safety valve spool 148 is communicated with a drain207 through a conduit 208 which connects the cavity 153 with the bore203, Communicating cavity 153 with drain 207 allows safety valve spring147 to force valve spool 148 to the right a distance sufficient to blockthe flow of fluid from differential valve 16. The blocking of fluid flowfrom the differential valve causes the valve spool 66 to move leftwardlywhich results in the disengagement of the selected directional clutch,which has the effect of neutralizing the transmission.

When the cavity 153 communicates with drain 207 the rightwardly mostedge 211 of safety valve spool 148 and the land edge 212 operate tometer fluid which passes into cavity 153 via conduit 47, recess 61,cross-drilled holes 213, and a central hole 214 in the valve spool 148.This fluid flow prevents full travel of the spool 148 to the right suchthat the valve is not reset to the full safety position and can recoverwhen the brake is released since.

the holes 213 communicate with the annulus 61. Thus, when the brakepedal is released, pressure is built up behind the safety valve and itmoves leftwardly allowing fluid to be routed to the directional clutchesthrough the differential valve whereby the transmission is reengaged.Thus, the safety valve works differently in response to transmissionneutralization than it does in response to an attempt to start thevehicle While the transmission is in gear. This is due to the fact thatthe spool 148 moves all the way to stop 151 when the vehicle has beenshut down, thus requiring the gear selector to be positioned in neutralin order to reestablish fluid flow to the selected differential clutch,While the spool 148 moves rightwardly only to the land edge 212 inresponse to neutralization. By not moving all the way to the stop 151during neutralization, the spool 148 is positioned such that the drilledholes 213 are continuously in communiaction with a source of fluidmaking it possible for fluid pressure to be reestablished in cavity 153without requiring a shift to neutral.

We claim:

1. A transmission hydraulic control system comprising in combination:

a source of pressure fluid;

a first set of hydraulically actuated friction engaging means;

a first selector valve means hydraulically connected between said sourceand said first set of friction engaging means, said valve means having afirst position in which fluid is directed to a first one of saidfriction engaging means and a second position in which fluid is directedto a second one of said friction engaging means;

a second set of hydraulically actuated friction engaging means; a secondselector valve means hydraulically connected between said source andsaid second set of friction engaging means, said second selector valvemeans having at least a first and second position for directing fluid todifferent ones of said second set of friction engaging means;

blocker means associated with both said first and said second selectorvalve means and operable to prevent said second valve means fromassuming the second position when said first selector valve means is inits first position and further operable to urge said second selectorvalve means to its first position from its second position in responseto a change of said first selector valve means from its second positionto its first position.

2. The control system of claim 1 wherein said second selector valvemeans has a third and fourth position and further comprises:

inhibitor means associated with said second selector valve means andoperable to inhibit changes of said valve from its first to its thirdposition without inhibiting a change from its third to its fourthposition.

3. The transmission hydraulic control system of claim 1 wherein saidsecond valve means includes a valve spool slidably disposed within avalve bore and said blocker means comprises piston means slidablydisposed in the valve bore and hydraulically urged toward said valvespool in response to said first selector valve means being in its firstposition.

References Cited UNITED STATES PATENTS 2,599,632 6/1952 Hindmarch19287.13 3,181,394 5/1965 Ramsel et a1. 74-754 BENJAMIN W. WYCHE III,Primary Examiner.

US. Cl. X.R. 192-333, 4

